Vinyl chloride polymers (PVC) are widely used for the manufacture of a variety of articles. They are used, for example, for the manufacture of sheets—generally reinforced with reinforcing fibers—intended for covering soils, for covering vehicles (trucks), for the concealing of buildings on which work is being carried out, for the construction of sets for entertainments or exhibitions, or for advertizing purposes. These sheets generally have a large surface area. Furthermore, in many applications, most particularly in advertising applications or in applications involving sets for entertainments or exhibitions, which at the present time are growing considerably, their lifetime is short, typically a few weeks or months. For these reasons, large quantities of such sheets are scrapped each year. The process of recycling them consequently constitutes a major ecological and economic problem.
The same situation applies with regard to other articles based on one or more, flexible or rigid, polymers (preferably PVC) for example with regard to conveyor belts, coated fabrics and other elements for the interior furnishing of vehicles, pipes and hoses, window frames or polymer-insulated power cables.
Thorough grinding of these articles usually results in a mixture of fine particles of heterogeneous composition, which is difficult to purify and reuse. Furthermore, in the case of fiber-reinforced articles (for example, polyester or nylon-fiber-reinforced articles), the fibers often form a kind of wadding which makes it much more complicated to reuse the ground particles.
Various processes based on dissolving, using organic solvents, have already been proposed; however, they often cause safety and pollution problems.
Patents EP 945481, EP 1232204 and EP 1268628 to SOLVAY aim at solving that problem by providing a recycling process which is simple, economic and reliable, which does not produce much pollution and which allows plastics of high purity and advantageous morphology to be recovered, while substantially preventing any additives from being extracted therefrom. This process includes the dissolution of the polymer in an adequate solvent capable of dissolving it and susceptible of forming an azeotrope with water, and its precipitation by injecting steam in the solution thus obtained, which moreover leads to the stripping of the water-solvent azeotrope and so leaves behind a mixture essentially consisting of water and of solid polymer particles which are recovered, while the vapors resulting from the stripping are condensed and decanted so as to recover both a solvent phase for a subsequent dissolution and a “water” phase for a subsequent precipitation.
These patents specify that if the article is reinforced with fibers, said fibers may be recovered as well and in order to increase their purity, they may be subjected to a subsequent centrifuging and/or washing step, for example using the same solvent, for the purpose of removing any residual traces of polymer. According to their teaching, the solvent which would have been used for the washing may advantageously be mixed with the fresh solvent used for the dissolving step; the fact that it contains traces of dissolved polymer does not in any way lessen the effectiveness of the dissolving operation. The fibers may be reused directly for the manufacture of plastic-based reinforced articles.
JP 2008-062186 describes in its examples, a washing treatment according to which the recovered fibers are first washed by “filtration washing” (i.e. the fibers are recovered from the polymer solution on a filter through which solvent is passed, eventually several times) and then, they are either (1) merely dried with hot air, or (2) they are first dispersed again in solvent to form a slurry in which steam is injected to remove the solvent, after which the fibers are recovered by filtration and dried using hot air. However, such a process offers the disadvantage that either the residual content of solvent on the fibers is too high (alternative (1)) or the process is economically unattractive because much solvent has to be evaporated (alternative (2)) or because of the use of hot air (alternatives (1) and (2)). Additionally, in both alternatives, the residual polymer of vinyl chloride is removed by rinsing with solvent through the fibers retained on a filter, which is not very efficient.